From the Proceedings of the 1971 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 22-23, 1971, Tempe, Arizona / The rotadisk rainulator is a recently developed rainfall simulator utilizing a full-cone-spray type nozzle. Its unique feature is the rotation of disks of various size openings that makes it possible to produce intensities from close to zero up to full nozzle capacity. Disks may be quickly changed, making it possible to study the effects of various intensities on infiltration rates, such as occur in natural storms. For all intensities above 1.0 in/hr, the instrument comes closer to duplicating kinetic energies and momenta of natural rainfall than any other type of rainfall simulator. Little rainfall-runoff data are available on most of the Lower Gila watersheds. Infiltration rates were therefore determined using the rotadisk rainulator on recompacted soil samples from the watershed. The results permitted a ranking of the watersheds on the basis of infiltration rates, which supports an independent flood frequency analysis indicating that the flood threat from subwatersheds along the Gila is much lower than had previously been projected. When the instrument is taken into the field, it should be possible to directly determine the infiltration rates of different soil and vegetation types, which will be of more use to hydrologists than data from recompacted samples